Indicator and control system



June 9, 1964 J. L. CROSTHWAIT 3,136,876

INDICATOR AND CONTROL SYSTEM Filed Oct. 26, 1960 TO AMPLIFIER TO SERVOMOTOR TO SENSING TO REFERENCE con. 30 con. 32

H 54 M 2 L L l c 0 RFC T T RFC lAMPLlFlEiI INVENTOR.

F. G .2 BY JEVON LCROSTHWAIT ATTORNEY 3,136,876 lllDlCA'ltBEl AND CONTRQL SYSTEM .levon L. Crosthwait, Newton Highlands, Mass, assignor to Clevite @orporation, a corporation of flhio Filed (Oct. 25, 196%, Ser. No. 65,231 16 (Ilaims. (Cl. 219--10.77)

This invention relates generally to systems for controlling, orindicating variations in, parameters effecting electromagnetic inductive coupling. The invention pertains also to induction heating apparatus embodying such con trol and indicator systems, and specifically contemplates its application in zone melting apparatus, particularly vertical floating-zone type furnaces such as are used for melting and recrystallizing elongate cylindrical ingots of polycrystalline semiconductor materials to produce similarly shaped single crystals of predetermined diameter for ultimate fabrication into semiconductor devices.

inasmuch as the invention is deemedat the present time to have particular advantage and utility in its application to floating-zone furnaces, it is herein described in this environment. It will be understood, however, that this is solely by Way of example rather than limitation; additional applications are mentioned at the conclusion of the detailed description of the exemplary embodiment and others may manifest themselves to persons skilled in the art. i

As is well-known in the art, the floating-zone technique and apparatus for carrying it out involve the disposition of an elongated bar of the fusible material being handled in a vertical position; establishing a molten zone at one end of the bar, usually by means of high frequency induction heating coils coaxially surrounding the bar; and

producing relative movement between the bar and coils so that the molten zone travels slowly along the entire length of the bar. Usually, the molten zone is moved upwardly from the bottom to the top of the bar and, if necessary or desirable, a single crystal seed is placed in contact with the end of the bar at Which the molten zone is initiated in order to promote single crystal growth.

The salient characterizing feature of the floating-zone procedure is the fact that, as the name implies, the molten zone is not provided with any external support; it is sustained solely and entirely by the characteristic surface tension of the fused material. As a result the material crystallizing at the trailing boundary (liquid/solid interface) of the zone, does so in the complete absence of any external physical contact which might exert a stress on the freezing material and cause polycrystalline growth.

It is, of course, highly desirable in growing single crys- United States Patent uniform cross-section.

Still another object is the provision of improved floating-zone type apparatus capable of growing bar crystals of a uniform cross-sectional dimension closely approaching the maximum limit imposed by the surface tension of the material crystallized.

A further object is the provision of an improved crystal size control system for floating-zone type crystallizing apparatus which is characterized by enhanced simplicity, sensitivity and reliability and which enables crystal diameter to be determined, within the limits imposed by surface tension of the material, to suit production convenience and economy for the particular size of the dice required for the device into which the crystal ultimately will be incorporated.

A still further object is the provision of a novel crystal size control system for floating-zone apparatus which is unaffected by frequency fluctuations in the service power supply.

These and other objects are fulfilled by apparatus in accordance with the present invention comprising an induction coil adapted to have a nominally conductive (i.e., conductive or. semiconductive) object disposed in its flux field. A sensing coil and reference coil are coaxially disposed with respect to the inductioncoil and are so dimensioned and located relative to each other and to the induction coil and such an object in its flux field that there is induced, in the sensing coil an electrical voltage due to, and by means of, inductive coupling to both the intale for semiconductor device fabrication to obtain a uniform cross-section in the interests of convenience and improved yield in processing into dice or wafers. Moreover, dimensional uniformity reduces variations in structural perfection of the crystal and in concentration of doping additions. Control of lateral dimensions has an additional importance where a floating-zone type operation is involved because each material operated upon has a characteristic maximum diameter beyond which the surface tension is not suflicient to sustain the molten zone. Therefore, to obtain a crystal of maximum possible cros section, desirable for economic reasons, Without risk of exceeding the critical limit requires delicate and precise control of the diameter of the molten zone which is, in turn, closely inter-related with that of the grown crystal.

With the foregoing circumstances in view, it is the fundamental general object of the present invention to provide improved monitoring and control systems for parameters effecting electromagnetic induction.

Another general object is the provision of improved induction heating apparatus including a monitoring and/ or duction coil and the alternating currents induced thereby in such object and, in the reference coil, an electrical voltage substantially due only to, and by means of, inductive coupling to the induction coil. Circuit means are provided for. comparing the respective electrical voltages in the sensing coil and reference coil to produce a difference signal indicative of changes in the inductive coupling between the sensingcoil and such an object and, therefore, of deviations by a parameter effecting inductive coupling from an established norm.

In accordance with an additional feature of the invention the signal generated in response to deviations of the parameter is utilized to correct the deviation.

Additional objects and advantages of the invention, its scope and the manner in which it may be practiced will be more fully apparent to those conversant with the art from a reading of the following detailed description of an exemplary embodiment thereof taken in conjunction with the subjoined claims and the annexed drawings in which FIGURE 1 is a schematic side elevational view, partially in section, illustrating the essential components of, and FlGURE 2 is a circuit "wiring diagram for, fioating-zone apparatus in accordance with the present invention.

In the interests of clarity and for literary ease the invention will be described as applied to the growth of a single crystal of semiconductor material, viz., silicon, but it will be understood that it may be applied with advantage to the crystallization of other semiconductors, as

. well as to zone melting in general, regardless of the maa transient molten zone; and the recrystallized monocrystalline ingot of silicon. While charge bar Ida is illustrated as being cylindrical this shape is not essential. It may be of other non-cylindrical, geometric or irregular configurations and it may have a cross-sectional dimension substantially different from recrystallized ingot lllc. It is to such situation that the present invention can be applied with particular utility and advantage.

For the purposes of this description, charge bar Ida and grown crystal ltlc may be considered as a monolithic bar of silicon (designated in its entirety by reference number along which molten zone ltlb is slowly traversed, the direction of travel assumed to be upward in the illustrated embodiment. The solid-liquid interfaces ltla-b and ltlb-c, respectively contiguous to the charge bar and the grown crystal, will be referred to as the leading and trailing interfaces, respectively, with allusion to the direction of zone travel.

The respective ends of bar It? are fixedly engaged at vertically-spaced locations by suitable supports which hold the bar in a vertical position. Thus a lower support 12 holds grown crystal portion 100 of the bar and an upper support 14 holds the charge portion 10a of the bar.

The vertical spacing between supports 12 and 14 is rendered adjustable by providing means for moving the supports vertically relative to one another, i.e., either or both of the supports may be movable. In the illustrated embodiment, lower support 12 is fixed, and upper support 14 is movable by means of a worm gear 16 driven by a reversible electric servo-motor 18 through a suitable gear train 19.

Molten zone it?!) is created, maintained, and progressively displaced along bar lltl by means of an induction heating coil 20 coaxially disposed with respect to the longitudinal axis of bar ill. Coil 2% is connected as by leads 22 to a suitable source of high frequency A.-C. power represented by radio frequency generator 24.

The induction heating system also includes a pair of short-circuited coils 26, 23, respectively disposed above and below, and coaxial with, coil 26, at relatively short distances therefrom. Coils 26 and 28 serve to constrict the magnetic field generated by induction coil 20 and, therefore, define the length of the molten zone.

The design parameters of induction coil heating system 29, 22, 24, 26, 28 insofar as they effect the axial extent of the molten zone produced, its temperatures, and such variables are well known in the art and form no part of the present disclosure.

Suffice it to say that the induction heating system produces a molten zone encompassing a relatively short (as compared to the total length of bar ltl) segment of the bar.

The structure thus far described is, generally speaking, conventional and typical of floating-zone furnaces. it will be appreciated that as the coil assembly Zll, 26, 28 traverses its vertical path along bar it molten zone ltlb concomitantly advances along the bar, the charge ltla fusing at leading interface ltla-b and the crystal 10c freezing at trailing interface ltlb-c.

In its movement, zone 10b maintains a characteristic ogee profile configuration. The concave curve 10d of the ogee is at a necked down region of minimum crosssection slightly above the mid-point of the zone; the convex curve 103 of the ogee encompasses the bottom portion of the molten zone adjacent trailing interface lltlb-c;

From a consideration of the drawing it will be appreciated that, all other variables being constant, the specific geometry or" the ogee at a given instant can be changed by regulating the axial strain extant in the zone.

This may be accomplished by varying the distance between interfaces itla-b and lilbw, in effect varying the length of zone 10b. When the interfaces 1tla-b and ltlb-c are moved toward each other, the convexly-curved segment ltle of the zone bulges outwardly beyond the circumference of interface 1@bc, tending to increase the diameter of the growing crystal. Conversely, when the distance between the respective interfaces is increased, lengthening the molten zone, the zone constricts radially so that the circumference of convexly-curved zone portion the is smaller than that of interface ltlb-c, the diameter of the growing crystal tends to decrease. It will be understood, of course, that the axial length of zone 1% under equilibrium conditions is substantially constant and is determined by design and functioning of the induction coil heating system 20-25; the changes in zone length effected by relative physical displacement of interfaces like-b and lltlb-c is evanescent in nature as is the resultant change in zone geometry.

In order to maintain a constant diameter in the grown crystal, the curve bounding zone portion file must be tangent to the sides of crystal 100. In accordance with the present invention this condition of tangencyis maintained by continuous monitoring of the cross-section of the grown crystal in the region of interface b-c to detect changes resulting from deviations in the desired tangency and applying a corrective adjustment to the spacing between interfaces lltlab and 1012-0.

To this end a single-turn sensing coil 38 is coaxially disposed about bar it) in close proximity to interface ltlb-c, being sufficiently close thereto that R.-F. alternating currents induced in the bar by coil 2% induce a potential in the sensing coil. An increase in the diameter of bar 2% at the location circumscribed by sensing coil 30 increases the degree of coupling between the bar and the sensing coil and, therefore, the magnitude of the potential induced in the latter. In like manner, a decrease in diameter has the converse effect.

Sensing coil 36 is disposed at a fixed axial distance from coil 29 (behind the coil as regards the direction of travel) and is movable therewith. The distance between sensing coil 30 and induction coil Ztl is small enough that an R.-F. voltage is induced in the former, independently of that resulting from the coupling to bar ltl. In a practical embodiment sensing coil 30 could be mechanically connected to the transport system, not shown, which moves the induction coil, and at such a distance therefrom that it is located at all times substantially in the plane of trailing interface ltlb-c.

Coaxially disposed with respect to induction heating coil 2% at a greater distance from bar it than sensing coil 39 is an additional single turn pickup coil 32, hereinafter termed the reference coil. The distance between induction coil 20 and reference coil 32 is small enough to allow effective electromagnetic coupling therebetween wtih the result that an R.-F. potential is induced in thelatter similar, at least in order of magnitude, to the R.-F. voltage induced in sensing coil 36 by virtue of its coupling to the induction coil as. On the other hand, reference coil 32 is far enough away from bar it) that inductive coupling thereto is negligible. The spacing conditions required are conveniently obtained, as in the illustrated embodiment, by mahng reference coil 32 of considerably larger diameter than sensing coil 3%; the desired degree of coupling between induction coil 26 and the reference coil then may be attained by selection of the axial spacing therebetween.

From the structure thus far described it will be seen that an R.-F. voltage of similar magnitude will be induced in both sensing coil 39 and reference coil 32 by virtue of inductive coupling to induction heating coil 24!. In addition there is induced in sensing coil 3% and R.-F. voltage proportional to the diameter of bar lid in the vicinity of interface lltlb-c. Assuming that the fixed voltages (i.e., those due to coupling to induction coil 2%), in the respective pickup coils are equal, the dilference in the total induced voltages reflects changes in the sensing coil voltage due and proportional to dimensional variations of the bar in the vicinity of interface lilb-c.

In accordance with the present invention, the voltages of sensing coil 36 and reference coil 32 are compared so as to generate a control signal proportional to the deviation of the cross-sectional dimension of the grown crystal at interface b-c from a pro-established value. This function is carried out by a circuit such as illustrated in FIGURE 2. The circuit, basically, comprises a load impedance and two substantially identical networks 36 and 38 for rectifying and attenuating the respective voltagesof the sensing and reference coils and applying them in opposition to one another across the load impedarrce so as to produce therein a current proportional to the voltage difference. This circuit will now be de scribed in greater detail with reference to the FIGURE 2 wiring diagram.

The load impedance of the circuit is represented by resistorR connected in'series with a center zero ammeter M. The series combination of meter M and load resistor R is common to both loops 36 and as, each of which comprises respective crystal diodes 40 and 42, arranged with opposite polarities relative to the load.

Thus, one terminal of sensing coil 30 is connected via a conductor 44 and a capacitive attenuator consisting of C and C to the positive terminal of diode 40 the negative side of which is connected through a dropping resistor R to one end 46 of the common load branch of networks 36 and 38. Similarly, one terminal of reference coil 32 is connected via a conductor 48 and a. capacitive attenuator consisting of C and C to the negative side of diode 42 the positive terminal of which is connected through variable resistor R to the same end 46 of the common branch. The respective remaining terminals of sensing coil 30 and reference coil 32 are returned to the other end 50 of the common branch by conductors 52 and 54.

By-pass capacitors C3 and C are connected across networks 36 and 38 in parallel with the common branch and, respectively, resistors R and R Radio frequency chokes L and L are connected across capacitors C and C respectively, to provide a return path for rectified D.-C. generated by the rectifying action of diodes 40 and 42.

In one practical embodiment of the circuit values employed were as follows:

(This is used as a coarse balance L=L=l mh. for 3 mo. operation. M=25-0-25 aa. meter.

- It will be seen from the wiring diagram that the respective polarities of the diodes 40 and 42 are reversed (relative to the common branch of networks 36 and 33) with the result that the rectified D.-C. potentials applied to the load impedance are in opposition,

A high gain amplifier 56 is connected across load resistance R fier 56 is fed to, and utilized to control the operation of, servo-motor 18. By adjustment of variable resistor R the voltages in the sensing coil and pickup coil, respectively, due to inductive coupling to the induction coil can be precisely balanced so that, for a given diameter of crystal 10c, i.e., the condition where the convex curvature of the molten zone is tangent to the sides of the grown crystal, no current flows in the load impedance branch of the circuit, the equal and opposite voltages producing a null.

In other words, R is adjusted to obtain a null when the crystal has the desired diameter (which can be achieved initially by manual control of motor 18,); Upon deviation in the diameter of the crystal at interface 1015-0, increasing or decreasing the total potential in Via conductors 58 the output of ampli-' the sensing coil, the null will be upset resulting in a current flowing in the load impedance generating an error signal across resistor R The error signal is amplified by amplifier 56 and applied to servo-motor 18 to actuate it. The direction of operation of the motor depends on the polarity of the signal, the relation between polarity and motor direction being selected so that a signal indicative of an excess diameter in the grown crystal operates the motor to increase the distance between upper and lower supports 12 and M- thus increasing the length of molten zone ltlb. Conversely, a signal indicative of a deficiency in the interface dimension causes reverse operation of motor 18 so that the upper support is moved toward the lower decreasing the length of the molten zone.

Gearing 19 of the servo-motor is selected in relation to the rate of growth of the crystal so that a correction to the crystal dimension is applied smoothly and without any tendency to spilling of the molten zone.

If desired amplifier 56 may be omitted and servo-motor 18 operated manually in accordance with visual observation of center zero meter M as required to maintain a substantially zero reading thereon.

It will be appreciated that spurious changes in frequency due to supply voltage variations and thermal effects in the induction generator have no effect on the operation because such changes are reflected in both the sensing coil and the reference coil and, consequently, are cancelled out in control circuit 34. Moreover, the system is highly sensitive only to the small region in the immediate vicinity of the interface ltlb-c so that the error signal is not affected by possible variations in the diameter of the unmelted charge 10a.

While the invention has been described, by way of example, as applied to the control of size in a floating zone type crystallizer, it will be understood that the basic principles involved are susceptible of diverse other applications.

Thus, the apparatus described utilizes the fact that a voltage is induced in sensing coil 3t) which is a function of the size of the crystal. .However, the voltage induced is also a function of the resistivity of the object in the field of the R.-F. induction coil. In the apparatus described, resistivity is virtually constant because the factors effecting resistivity, e.g., the identity of the material, and its temperature also are essentially constant, and the variable to be controlled in size. However, the same basic principles and general arrangement can be utilized to measure or control one of the other variables, e.g., temperature Where size is constant. One application of this type'would be to brazing apparatus employing an R.-F. induction heater. Provided that the object to be brazed is composed of a material having an appreciable temperature coefficient of resistivity as is usually the case, the sensing arrangement described hereinabove could be incorporated as a temperature indicator or controller.

Therefore, while there have been described what at present is believed to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the basic principles involved, and it is aimed, therefore, to cover in the appended claims all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimedand desired to be secured by United States Letters Patent is:

l. A monitoring and :control system for parameters effecting electromagnetic inductive coupling, comprising: an induction coil adapted to have a nominally conductive object disposed in its fiux field; a sensing coil and a reference coil coaxially disposed with respect to said inducdue to, and by means of, inductive coupling to both said induction coil and the alternating currents induced thereby in such object and, in the reference coil, an electrical voltage substantially due only to, and b means of, inductive coupling to said induction coil; and circuit means or comparing the respective electrical voltages in said sensing coil and reference coil, to produce an electrical difference signal indicative of changes in the inductive coupling between said sensing coil and such an object and, therefore, of deviations by a parameter effecting said coupling from an established norm.

2. Apparatus according to claim 1 wherein said circuit means comprises a load impedance; means for rectifying the respective voltages induced in said sensing coil and said reference coil to obtain unidirectional voltages applied, in opposition to each other, across said load impedance to produce therein a current proportional to the difference between said unidirectional voltages, the potential drop across said load impedance constituting said difference signal.

3. Apparatus according to claim 2 wherein said circuit means includes means for adjusting the unidirectional voltage derived from said reference coil to a predetermined value correlated to a particular value of said parameter.

4. Induction heating apparatus comprising: a high frequency electrical induction heating coil efiective to heat an object in its flux field; a sensing coil coaxially disposed with respect to said heating coil, the size of said sensing coil and its location relative to said heating coil and to an object in the flux field of said heating coil being such that there is induced in said sensing coil an electrical voltage due to and by means of inductive coupling to high frequency alternating current produced in such an object by said heating coil, the magnitude of said voltage being directly proportional to the degree of said inductive coupling which, in turn, is variable in accordance with the proximity between such object and said sensing coil and with the resistivity of such object and, therefore, its temperature; and means for establishing a reference voltage and comparing thereto the induced voltage in the sensing coil to generate an electrical signal indicative of deviations of temperature, size and location of such an object from a pro-established datum. I

5. induction heating apparatus comprising: a high frequency electrical induction heating coil effective to heat an object in its flux field; a sensing coil coaxially disposed with respect to said heating coilfthe size of said heating coil and its location relative to said heating coil and to an object in the flux field of said heating coil being such that there is induced in said sensing coil an electrical voltage due to and by means of inductive coupling to high frequency alternating current produced in such an object by said heating coil, the magnitude of said voltage being directly proportional to the degree of said inductive coupling which, in turn, is variable in accordance with the proximity between such object and said sensing coil and with the resistivity of such object and, therefore, its temperature; means for establishing a reference voltage and comparing thereto the induced voltage in the sensing coil to generate a control signal; and means utilizing said control signal to adjust a parameter affecting the inductive coupling.

6. Induction heating apparatus comprising: a radio frequency electrical induction heating coil effective to heat an object in its flux field; a sensing coil coaxially disposed with respect to said heating coil, the size of said sensing coil and its location relative to said heating coil and to an object in the flux field of said heating coil being such that there is induced in the sensing coil an electrical voltage due to, and by means of, inductive coupling to both said induction coil and the high frequency alternating current product thereby in such object, the portion of said voltage due to current in such object being directly proportional to the degree of said inductive coupling which, in turn, is variable in accordance with the proximity between such object and said sensing coil and with the resistivity of such object and, therefore, its temperature; an additional coil disposed at a location sulficiently remote from an object in the flux field of the heating coil that inductive coupling thereof to such an object is of negligible effect and sufiiciently close to said heating coil to cause a substantially constant reference voltage to be induced in said additional coil; circuit means for comparing the induced voltage in said sensing coil to said reference voltage and generating an electrical signal indicative of the difference therebetween.

7. Induction heating apparatus according to claim 5 including means for utilizing said signal to adjust a parameter effecting the inductive coupling between said sensing coil and such object.

8. induction heating apparatus comprising: high frequency electrical induction heating coil means effective to heat an object disposed in its flux field; sensing coil means coaxially disposed with respect to said heating coil means, the size of said sensing coil means and its location relative to said heating coil means and to an object in the flux field of said heating coil means being such that there is induced in the sensing coil means an electrical voltage due to, and by means of, inductive coupling to both said induction heating coil means and the high frequency alternating current produced thereby in such object, the portion of said voltage due to current in such object being directly proportional to the degree of said inductive coupling which, in turn, is variable in accordance with the proximity between such object and said sensing coil means and with the resistivity of such object and, therefore, its temperature; additional coil means coaxially disposed with respect to said heating coil means at a location which normally would be sufficiently more remote than said sensing coil from such an object that inductive coupling thereof to such object is of negligible effect, said additional coil means being suificiently close to said heating coil means to cause a substantially constant reference voltage to be induced in the additional coil means; circuit means for comparing the induced voltage in said sensing coil means to said reference voltage to generate a control signal; and means responsive to said control signal for varying a parameter effecting the inductive coupling between such an object and said sensing coil means.

9. Apparatus for passing an unsupported molten zone along a vertically-disposed and elongated bar of fusible material, comprising: respective support means fixedly engaging said bar at vertically spaced locations; means operative to adjust the spacing between said support means; high frequency electrical induction heating means effective to create a molten zone in said bar encompassing a relatively short longitudinal segment thereof; means for vertically displacing said heat-generating means relative to said bar with concomitant displacement of said molten zone therealong; a sensing coil, located at a constant vertical distance relative to said heat generating means, concentrically disposedabout said bar so as to be in close proximity to the liquid-solid interface at the trailing end of said molten zone whereby an induced electrical voltage is generated in said sensing coil by inductive coupling to high frequency alternating current produced in said bar by said heating means, the magnitude of said voltage being directly proportional to the degree of said inductive coupling which, in turn, is directly proportional to the proximity between said bar and said sensing coil and, therefore, to the cross-sectional dimension of said bar; and means for establishing a reference voltage and comparing thereto the induced voltage in the sensing means to generate a control signal proportional to the deviation of the cross-sectional dimension from a pre-established value.

10. Apparatus for passing an unsupported molten zone along a vertically-disposed and elongated bar of fusible material, comprising: respective support means fixedly engaging said bar at vertically spaced locations; means operative to adjust the spacing between said support means; high frequency electrical induction heating means effective to create a molten zone in said bar encompassing a relatively short longitudinal segment thereof; means for vertically displacing said heating means relative to said bar with concomitant displacement of said molten zone therealong; a sensing coil, located at a constant vertical distance relative to said heating means, concentrically disposed about said bar so as to be in close proximity to the liquid-solid interface at the trailing end of said molten zone whereby an induced electrical voltage is generated in said sensing coil by inductive coupling to high frequency alternating current produced in said bar by said heating means, the magnitude of said voltage being directly proportional to the degree of said inductive coupling which, in turn, is directly proportional to the proximity between said bar and said sensing coil and, therefore, to the crosssectional dimension of said bar; means for establishing a reference voltage and comparing thereto the induced voltage in the sensing coil to generate a control signal; and means utilizing said control signal to operate said adjustment means and thus vary the vertical spacing between said support means.

ll. Apparatus for passing an unsupported molten zone along a vertically-disposed and elongated bar of fusible material, comprising: respective support means fixedly I engaging said bar at vertically spaced locations; means operative to adjust the spacing between said support means; radio frequency electrical induction heating coil means effective to create a molten zone in said bar encompassing a relatively short longitudinal segment thereof; means for vertically displacing said induction heating coil means relative to said bar with concomitant displacement of said molten zone therealong; sensing coil means, located at a fixed vertical distance relative to said induction heating coil means, concentrically disposed about said bar so as to be in sufficiently close proximity to both said induction heating coil and the liquid-solid interface at the trailing end of said molten zone that there is induced in the sensing coil means an electrical voltage due to and by means of inductive coupling to both said induction coil means and the high frequency alternating current produced thereby in said bar, the portion of said voltage due to current in the bar being directly proportional to the degree of said inductive coupling which, in turn, is directly proportional to the proximity between said bar and said sensing coil means and, therefore, to the lateral dimension of said bar; additional coil means disposed about said bar at a location suf-, ficiently more remote from the bar than said sensing coil means that inductive coupling thereof to the bar is of negligible effect and sufficiently close to said induction coil to cause a substantially constant reference voltage to be induced in the additional coil means; circuit means for comparing the induced voltage in said sensing coil means to said reference voltage and generating a control signal related to the difference therebetween; and means utilizing said control signal to operate said adjustment means and thus vary the vertical spacing between said support means. i

12. Apparatus for passing an unsupported molten zone along a vertically-disposed and elongated bar of fusible material, comprisingz respective support means fixedly engaging said bar at vertically spaced locations; means operative to adjust the spacing between said support means; radio frequency electrical induction heating coil means effective to create a molten zone in said bar encompassing a relatively short longitudinal segment thereof; means for causing relative axial displacement between said induction coil means and said bar with concomitant displacement of said molten zone along the bar; sensing coil means located at a fixed vertical distance relative to said induction heating coil, concentrically disposed about said bar so as to bein sufficiently close proximity to both said induction heating coil means and the liquid-solid interface at the trailing end of said molten zone that there is induced in the sensing coil means an electrical voltage due to and by means of inductive coupling to both said induction heating coil means and the high frequency alternating current produced thereby in said bar, the portion of said voltage due to current in the bar being directly proportional to the degree of said inductive coupling which, in turn, is directly proportional to the proximity between said bar'and said sensing coil. means and, therefore, to the lateral dimension of said bar; additional coil means disposed about said bar at a location sufliciently more remote from the bar than said sensing coil means that in ductive coupling thereof to the bar is of negligible effect and sufficiently close to said induction coil to cause a substantially constant reference voltageto be induced in the additional coil means; circuit means for comparing the induced voltage in said sensing coil means to said reference voltage; means included in said circuit means for adjusting said reference voltage to establish a null at a predetermined value of said induced voltage in the sensing coil and to generate a control signal upon the occurrence, and in proportion to the magnitude, of deviations from said null; and means utilizing said control signal to operate said spacing adjustment means and thus vary the vertical spacing between said support means.

13. Apparatus for passing an unsupported molten zone along a vertically-disposed and elongated bar of fusible material, comprising: respective support means fixedly engaging said bar at vertically spaced locations; means operative to adjust the spacing between said support means; radio frequency electrical induction heating coil means effective to create a molten zone in said bar encompassing a relatively short longitudinal segment thereof; means for causing vertical displacement between said induction heating coil means and said bar with. concomitant displacement of said molten zone along the bar; sensing coil means, located at a fixed vertical distance relative to said induction heating coil means, concentrically disposed about said bar in substantially the same plane as and in sufficiently close proximity to the liquid-solid interface at the trailing end of saidmolten zone that there is induced in the sensing coil means an electrical voltage due to and by means of inductive coupling to both said induction heating coil means and the high frequency alternating current produced thereby in said bar, the portion of said voltage due to current in the bar being directly proportional to the degree of said inductive coupling which, in turn, is directly proportional to the proximity between said bar and said sensing coil means and, therefore, to the lateral dimension of said bar; additional coil means disposed about said bar at a location sufficiently more remote from the bar than said sensing coil means that inductive coupling thereof to the bar is of negligible effect and sufiiciently close to said induction coil to cause a substantially constant reference voltage to be induced in the additional coil means; circuit means for comparing the induced voltage in said sensing coil means to said reference voltage and generating a control signal related to the difference therebetween; and means utilizing said control signal to operate said spacing adjustment means and thus vary the vertical spacing between said support means.

14. Apparatus according to claim 13 wherein said circuit means comprises a load impedance; means for rectifying the respective voltages induced in said sensing coil means and said additional coil means to obtain unidirectional voltages applied, in opposition to each other, across said load impedance to produce therein at current proportional to the difference between said unidirectional voltages, the potential drop across said load impedance constituting said control signal.

15. Apparatus according to claim 14, wherein said cir- 1 1 cuit means includes means for adjusting the unidirectional voltage derived from said additional coil means to a predetermined value correlated to a particular dimension of said bar at said interface.

16. An induction heating apparatus for a movable object havingna variable, dimension comprising: a high frequency electrical induction heating coil effective to heat an object in its flux field; a sensing coil coaxially dis-, posed with respect to said heating coil and positioned adjacent the object whereby a voltage is induced in said sensing coil as a result of inductive coupling with the object and as a result of inductive coupling with said heating coil; a reference coil coaxially disposed with respect to said heating coil to be inductively coupled only thereto whereby a voltage is induced in said reference coil as a result of inductive coupling thereof with said heating coil; and circuit means for comparing the electrical voltages induced in said sensing coil and said reference coil respectively to produce an electrical difier- .Lufi! ence signal indicative of the changes in inductive coupling between said sensing coil and the object and thus indicative of variations in dimension of the object.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Germany application, 1,022,698, Jan. 16, 1958. 

16. AN INDUCTION HEATING APPARATUS FOR A MOVABLE OBJECT HAVING A VARIABLE DIMENSION COMPRISING: A HIGH FREQUENCY ELECTRICAL INDUCTION HEATING COIL EFFECTIVE TO HEAT AN OBJECT IN ITS FLUX FIELD; A SENSING COIL COAXIALLY DISPOSED WITH RESPECT TO SAID HEATING COIL AND POSITIONED ADJACENT THE OBJECT WHEREBY A VOLTAGE IS INDUCED IN SAID SENSING COIL AS A RESULT OF INDUCTIVE COUPLING WITH THE OBJECT AND AS A RESULT OF INDUCTIVE COUPLING WITH SAID HEATING COIL; A REFERENCE COIL COAXIALLY DISPOSED WITH RESPECT TO SAID HEATING COIL TO BE INDUCTIVELY COUPLED ONLY THERETO WHEREBY A VOLTAGE IS INDUCED IN SAID REFERENCE COIL AS A RESULT OF INDUCTIVE COUPLING THEREOF WITH SAID HEATING COIL; AND CIRCUIT MEANS FOR COMPARING THE ELECTRICAL VOLTAGES INDUCED IN SAID SENSING COIL AND SAID REFERENCE COIL RESPECTIVELY TO PRODUCE AN ELECTRICAL DIFFERENCE SIGNAL INDICATIVE OF THE CHANGES IN INDUCTIVE COUPLING BETWEEN SAID SENSING COIL AND THE OBJECT AND THUS INDICATIVE OF VARIATIONS IN DIMENSION OF THE OBJECT. 